This invention relates generally to wireless communications systems employing digital control and traffic channels and, more particularly to digital wireless communications systems capable of transmitting packet data.
An Enhanced Global Packet Radio System (EGRPS) Compact control channel solution introduces a discontinuously transmitting rotating control channel. This control channel solution makes it possible to deploy a GSM system with less than a one MHz bandwidth in a synchronous network. The Compact control channel solution proposals were adopted at UWCC.GTF.PDFG/RF-group meeting May 13-15, 1999 in Monterey (UWCC.GTF.PDFG/99.04.13.38, 3-Carrier Compact Proposal). The Compact control channel solution is a proposal for a TDMA, 136HS network that maintains a spectrum deployment below one MHz.
A general goal of the Compact solution is to use three carriers (600 kHz) in a ⅓ reuse manner, but to achieve a nine or twelve reuse factor for control channels by using a 200 kHz synchronous network. This 3-Carrier Compact Solution was presented to ETSI at the Paris EDGE SMG workshop (Tdoc SMG2 EDGE 122/99, Tdoc SMG2 EDGE 152/99, Tdoc SMG2 EDGE 153/99), incorporated by reference herein.
The Compact control channel does not use the conventional GSM 51-multiframe for control since the 51-multiframe includes all of the circuit switched broadcast support. Instead, what was selected was a packet control channel (Packet Broadcast Control Channel plus Packet Common Control Channel (PBCCH+PCCCH)) on a 52-multiframe basis. The existing 52-multiframe PBCCH control channel structure was modified to support as well a frequency correction and synchronization feature for mobile stations. One frequency correction burst (PFCCH) was added to the 26th frame, and one synchronization burst (PSCH) was added to the 52nd frame.
The inventors have realized that the 3-Carrier Compact Solution, as presently proposed, has several inherent problems.
First, since exactly the same frequency correction and synchronization bursts are used, as with current GSM 51-multiframe, a mobile station cannot differentiate the Compact 52-multiframe control channels from the conventional (or classic) GSM 51-multiframe control channels. As a result, there is potential that the operation of conventional mobile stations, which xe2x80x9cunderstandxe2x80x9d only the 51-multiframe control channel, will be disturbed by the new 52-multiframe compact control channel. This is possible since the mobile station searches for frequency correction bursts during its initial synchronization, and a conventional mobile station may locate and lock to a Compact control channel (which it is not compatible with). Even if the mobile station were not to lock to the Compact control channel, it can be expected that the cell selection procedure of the conventional mobile stations will become slower.
A second problem is that the initial cell selection procedure becomes challenging as well for xe2x80x9cnewxe2x80x9d mobile stations (i.e., those compatible with the 3-Carrier Compact Solution), since these mobile stations should be able to synchronize either to the conventional continuously transmitting 51-multiframe control channel or to the discontinuously transmitting 52-multiframe control channel. The mobile station begins the initial cell selection process by measuring the received signal level from each carrier of the band.
Since the Compact control channel is discontinuously transmitting, the mobile station must measure each channel long enough to insure that it receives at least one occurrence of the Compact control channel transmission. Due to the discontinuous nature of the transmission, the scanning time can increase from about five seconds to as much as about two minutes. After the scanning procedure the mobile station lists the channels of the band according to received signal level, and then attempts to synchronize with the control channels, starting at the top of the list (i.e., with the control channel having the greatest signal level). The most problematic case is when the mobile station attempts to synchronize as a first priority to a particular network which uses only compact control carriers, as the continuous control carriers from another network or networks will most probably dominate the control channel list after the received signal level scan.
In the conventional approach a Pure Sine Wave (PSW) search is used during the initial synchronization for adjusting the mobile station""s local oscillator frequency and to find a coarse synchronization channel (SCH) burst position, since a real time SCH burst search was not possible with existing hardware. The PSW is also a xe2x80x9clightxe2x80x9d algorithm in so far as the DSP is concerned.
When the conventional mobile station finds the PFCCH burst from the proposed Compact carrier, which is identical to the GSM FCH burst, the mobile station may remain tuned to that carrier for a long period of time. Since the mobile station does not find the SCH burst in the expected location, after assuming that it has located the FCH burst, it may determine that its frequency offset is not correct, and therefore a new FCH burst adjustment can be made. This problem can exist as well with the stored PLMN list search. For example, the mobile station may be switched off at one location, and then switched back on at another location. The stored last camped control channel and neighbor channel list information can be misleading at the second location, if a different network operator has placed the Compact control channel system side-by-side with the mobile station""s operator""s conventional (classical) control channel system.
In an attempt to solve the initial cell selection time problem, it has been proposed to introduce a different PSCH training sequence for the Compact control channel. However, this proposed solution has the drawback that it does not solve the problem related to the incompatibility issue with conventional mobile stations. In addition, the use of the proposed PSCH training sequence information requires much more Digital Signal Processor (DSP) processing power in the mobile station, as the mobile station must be able to simultaneously search the PFCCH and PSCH bursts. Furthermore, if the mobile station local oscillator exhibits a significant error during the period of initial cell selection, the PSCH search may not work at all.
It is a first object and advantage of this invention to provide a technique that is able to reduce an initial cell selection time.
It is another object and advantage of this invention to provide a technique to distinguish a continuously transmitted 51-multiframe control channel from a discontinuously transmitted 52-multiframe control channel, particularly one that employs Compact control channels.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
The foregoing and other problems are solved, in accordance with the teachings of this invention, by the introduction of an improved frequency correction burst in the 52-multiframe control channel. Instead of using exactly the same frequency correction format as with the conventional 51-multiframe approach, the format of the frequency correction burst is modified to make it distinguishable from the conventional 51-multiframe frequency correction burst.
More particularly, the 51-multiframe frequency correction burst is transmitted as all zeroes 0000 . . . which yields a 67.7 kHz pure sine wave (PSW) signal above the carrier frequency. In accordance with the teachings of this invention, a 1010 . . . bit pattern is used instead for the 52-multiframe control channel, which yields a pure sine wave (PSW) signal 67.7 kHz beneath or under the carrier frequency. Using this frequency correction format, the operation of conventional mobile stations is not interfered with, while new mobile stations are enabled to distinguish the conventional control channel from the Compact control channel. As a result, the initial cell selection process is enhanced and is executed in a reduced amount of time.
During the scan procedure the mobile station is able to distinguish the control channels as follows.
(A) If the received signal strength level (RX-level) is high (above some predetermined receive threshold), but neither the 67.7 kHz sine or the xe2x88x9267.7 kHz sine are found, then the current channel may be a traffic channel or some non-GSM channel.
(B) If the RX-level is high, and the 67.7 kHz sine is found, then the current channel is a conventional 51-multiframe continuous GSM control channel.
(C) If the RX-level is high, and the xe2x88x9267.7 kHz sine is found, then the current channel is a 52-multiframe discontinuous Compact control channel.
The mobile station is then enabled to use this information when attempting to synchronize to a most preferred Public Land Mobile Network (PLMN). The PLMN list is stored in the mobile station, such as in the Subscriber Identity Module (SIM), and provides information about control channel usage. For example, the first priority PLMN may use only compact control channels. In this case the mobile station is enabled to begin its synchronization search with those channels which were listed as being possible Compact control channels.
A method is thus provided to distinguish a first type of control channel from a second type of control channel. The method includes steps of (a) transmitting a carrier of the first type of control channel so as to include a first symbol sequence that results, when demodulated, in a sine wave having a frequency with a first offset from the carrier; (b) transmitting a carrier of the second type of control channel so as to include a second symbol sequence that results, when demodulated, in a sine wave having a frequency with a second offset from the carrier; and (c) demodulating a received carrier and detecting whether the carrier includes the first type of control channel of the second type of control channel.
In the preferred embodiment of this invention the first symbol sequence is an all zeroes sequence, and the second type of symbol sequence is an alternating ones and zeroes sequence, which results in the first offset being a positive offset, and the second offset being a negative offset. In a most preferred embodiment the first offset is +67.7 kHz, and the second offset is xe2x88x9267.7 kHz.
The step of demodulating includes a step of multiplying an I/Q representation of the received symbol sequence by 1xe2x88x92j, xe2x88x921 j, 1xe2x88x92j, . . . , and by 1 j, xe2x88x921xe2x88x92j, 1 j, . . .